Reflective sensor for detection of material degradation

ABSTRACT

A sensor for detecting material degradation may include an optical fiber and a housing through which the optical fiber extends. An end cap may be affixed to an end of the housing. Light provided through the optical fiber may be reflected off of the end cap back through the optical fiber. The end cap may be made of a material of interest, and may be situated in an environment wherein the material of interest is present. A light source may provide input light through the optical fiber. A portion of the input light may be reflected off of the end cap. A light receptor may receive the reflected light via the optical fiber. A processing unit may be adapted to compare a measured intensity of the reflected light to a threshold, and to initiate an alarm condition if the measured intensity is below the threshold.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims benefit under 35 U.S.C. §119 of provisional U.S.patent application No. 61/708,119, filed Oct. 1, 2012, the disclosure ofwhich is incorporated herein by reference.

BACKGROUND

In some applications, it may be desirable to detect the degradation ofcertain materials. For example, it may be desirable to detect corrosionof rebar in concrete to determine structural integrity, such as inroadways and bridges, for example. Rebar may be defined as a metal rodor bar used for reinforcement in concrete or asphalt pourings.

Near-infrared (NIR) analysis detects and measures the concentration ofchloride ions or other corrosion-causing substances in concrete, but itdoes not report whether the corrosion of rebar actually took place andto what extent. The presence of chloride, or other substances, such assulphates or carbonates, for example, may cause corrosion, but otherfactors also play a role, e.g., moisture, pH, the nature of the concreteand rebar, traffic-caused stress, etc.

It may be desirable, therefore, if there were available a simple,low-cost sensor that is capable of detecting and reporting theoccurrence and extent of material degradation, such as the corrosion ofrebar in bridge decks, for example.

SUMMARY

As disclosed herein, an example sensor for detecting materialdegradation may include an optical fiber and a housing through which theoptical fiber extends. An end cap may be affixed to an end of thehousing. Light provided through the optical fiber may be reflected offof the end cap back through the optical fiber. The end cap may be madeof a material of interest, and may be situated in an environment whereinthe material of interest is present.

A light source may provide input light through the optical fiber. Aportion of the input light may be reflected off of the end cap. A lightreceptor may receive the reflected light via the optical fiber. Aprocessing unit may be adapted to compare a measured intensity of thereflected light to a threshold, and to initiate an alarm condition ifthe measured intensity is below the threshold.

An example method for determining material degradation may includesituating an end of an optical fiber in proximity to a material. The endof the optical fiber may be abutted against the material, for example.Input light may be provided through the optical fiber such that at leasta portion of the input light is reflected off of the material asreflected light. An intensity of the reflected light may be measured.

From the measured intensity of the reflected light, it may be determinedwhether the material has degraded by more than an acceptable amount. Themeasured intensity of the reflected light may be compared to athreshold. The threshold may be based on a benchmark intensity of lightreflected off of the material before the material begins to degrade. Themeasured intensity of the reflected light may be compared to theintensity of the input light.

Another example sensor may include an optical fiber having an end thatis situated in proximity to a material. The material may be an end capsituated at the end of the optical fiber. The material may be a coatingapplied directly to the distal end of the fiber. A light source mayprovide input light through the optical fiber such that at least aportion of the input light is reflected off of the material as reflectedlight. A light receptor may receive the reflected light via the opticalfiber. A processing unit may be adapted to determine from a measuredintensity of the reflected light whether the material has degraded.

The processing unit may be adapted to measure the intensity of thereflected light, and to compare the measured intensity of the reflectedlight to a threshold. The processing unit may be adapted to initiate analarm condition if it is determined that the measured intensity is belowthe threshold, or if it is otherwise determined that the material hasdegraded by more than a predefined amount.

An optical time-domain reflectometer (OTDR) may be employed as the lightsource and light receptor. The OTDR may also be used to locate faults,such as breaks, for example, in an optical fiber. Use of an OTDR may beparticularly suitable where a network of fiber-optic sensors isdeployed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1C depict an example fiber-optic sensor for detecting materialdegradation.

FIG. 2 provides the results of an accelerated material degradation test.

FIG. 3 depicts example fiber-optic sensors imbedded in a concrete deck.

FIG. 4 illustrates an example placement of sensors in a test concretedeck.

FIG. 5 depicts an example sensor system including an optical time-domainreflectometer (OTDR).

FIG. 6 depicts an example sensor system based on wavelength detection.

FIG. 7 depicts an example sensor capsule with wireless communication.

DETAILED DESCRIPTION

FIGS. 1A-1C depict an example fiber-optic sensor 100. Such a sensor 100may be particularly suitable for detecting corrosion of rebar in aconcrete structure. It should be understood, however, that such a sensormay also be useful for detecting degradation of other materials, such aspaint, plastic, TEFLON, brake padding, or a noble metal, for example. Asused herein, the term “degradation” may be used liberally to refer toany change in the condition of a material, such as by degradation,deterioration, dissolution, corrosion, erosion, abrasion, oxidation,delamination, or other wear, for example.

As shown, the sensor 100 may include an optical fiber 102, a housing104, and an end cap 106. Optical fibers are well-known, and need not bedescribed in detail herein. Suffice it to say that the optical fiber 102may be made of any desirable optical material such that light may passthrough the optical fiber 102 in both directions from one end to theother. The optical fiber 102 may extend through the housing 104. Theoptical fiber 102 may be one of a plurality of optical fibers thatextends through the housing 104. Such a structure—a plurality of opticalfibers contained by a housing—may be referred to as an optical cable.

The housing 104 may be made of steel, for example. It should beunderstood, however, that the housing 104 may be made of any suitablematerial, such as iron, ceramic, glass, plastic, polymer, or any of anumber of various metals, for example. The housing 104 may be formed tohave the shape of a hollow cylinder. It should be understood, however,that the housing 104 may be formed to have any desirable shape, such asa flat plate or a box, for example. A box-shaped housing may beparticularly desirable where an arrangement of optical fibers isprovided within a single housing. Such an arrangement may be a linear ortwo-dimensional array of optical fibers, for example. An examplecylindrical housing 104 may have a length of about 12 mm and a radius ofabout 2.5 mm. It should be understood, however, that the housing 104 mayhave any desirable dimensions.

The end cap 106 may be made of any material that the sensor 100 isdesigned to monitor. For example, in an application where the sensor 100is designed to monitor rebar in a concrete installation, the end cap 106may be made of the same material as the rebar, e.g., steel.

The end cap 106 may be affixed to an end of the housing 104. The end cap106 may be affixed to the end of the housing 104 by any desirable means,such as by epoxying, gluing, welding, or brazing, for example. Theoptical fiber 102 may abut the end cap 106. It should be understood thatthe end cap 106 is optional, and that the distal end of the opticalfiber 102 may be situated in proximity to the material that the sensor100 is designed to monitor. For example, if the material to be monitoredis paint, the distal end of the optical fiber 102 may be abutted againstthe paint itself. Instead of an end cap, the distal end of the opticalfiber 104 may be coated directly with a layer of the material ofinterest. For example, the distal end of the optical fiber 104 may becoated with a layer of paint, or a metal may be sputter-coated onto theend of the fiber 104.

The optical fiber 104 may carry light 108E that is emitted from a lightsource S, through the optical fiber 102, to the end cap 106. The emittedlight 108E may be reflected off of the end cap 106 as reflected light108R, and travel back through the optical fiber 102, to a light receptorR. A processing unit, P, may be provided to measure an intensity of thereflected light. The processing unit P may be further adapted to comparethe measured intensity of the reflected light to a threshold. Theprocessing unit P may be further adapted to initiate an alarm conditionif the measured intensity is below the threshold.

Based on the intensity of the reflected light 108R, the sensor 100 candetermine the extent to which the end cap 106 has degraded. The sensor100 may determine whether the material from which the end cap 106 ismade has degraded by more than a certain amount by comparing theintensity of the reflected light 108R to a threshold. Thus, over time,when an insufficient amount of the emitted light is being reflectedback, the sensor 100 can conclude that the material has degraded toomuch. The threshold may be based on the intensity of the emitted light108E. Thus, the sensor could compare the intensity of the reflectedlight 108R to the intensity of the emitted light 108E. The threshold maybe based on a benchmark intensity of the reflected light 108R, i.e., anintensity of the reflected light 108R as measured before the sensor 100is placed into service. Thus, the sensor could compare the intensity ofthe reflected light 108R to its initial intensity before the materialbegan to degrade. The threshold may be preset based on the nature of thematerial being monitored. The threshold could be effectively zero. Thatis, the sensor could monitor the reflected light until the material hasdegraded sufficiently that no appreciable amount of light is reflectedback.

FIG. 1A depicts the sensor 100 during the early stages of its operationwith a newly installed end cap 106. In this example, the reflected light108R has a first (benchmark) intensity. FIG. 1B depicts the sensor 100at some point during its life with a degraded end cap 106A. In thisexample, the reflected light 108R has a second intensity that is lessthan its benchmark intensity due to degradation of the end cap 106. FIG.1C depicts the sensor 100 at a point in its life when the end cap hascompleted degraded away. In this example, there might be no appreciableamount of reflected light.

In a bridge application, where the sensor 100 is being employed todetect the degradation of rebar in concrete, the end cap 106 may be madeof steel. Light reflected off the end cap 106 may have a benchmarkintensity before the end cap 106 begins to degrade. When the end cap 106begins to degrade, the intensity of the reflected light 108R will bereduced relative to its benchmark intensity. Eventually, the end capwill degrade to a point where further degradation is no longertolerable. At that point, the reflected light 108R will have a thresholdintensity. When the sensor detects that the reflected light has anintensity that is less than the threshold intensity, the sensor canconclude that the end cap has degraded by an unacceptable amount.Because the end cap is made of the same material as the material beingmonitored (e.g., rebar in a concrete installation), it may be assumedthat the actual rebar has also degraded to below a certain, acceptablelevel. The thickness of cap may be chosen such that when the cap hascorroded away completely, it may be assumed that the correspondingmaterial in the installation has corroded more than an acceptable level.

Several sensors were tested by the accelerated corrosion in HClsolutions. FIG. 2 shows the results of an accelerated corrosion test inHCl. Prior to corrosion, the reflected light signal was very strong.After the onset of the corrosion, the reflected light signal was veryweak.

FIG. 2 provides the results of the accelerated corrosion test as afunction of reflectivity over time. In the test, the distal end of theoptical fiber was abutted against a 25-micron thick sheet of steel foil.The foil-tipped optical fiber was immersed in HCl. As shown in thegraph, the reflectivity remained relatively constant for about nineminutes after the foil-tipped optical fiber was immersed. After that,the onset of degradation could be detected as evidenced by the varyingreflectivity. It can be observed from the vacillations in thereflectivity between nine and 20 minutes that not only did the foildegrade, but also separated in distance from the tip of the opticalfiber. Thus, a sensor as described herein can also be used to detectseparation in distance between the optical fiber and the material ofinterest. After about 22 minutes, the reflectivity fell offsignificantly relative to benchmark. Eventually, when the foil wascompletely dissolved, the reflectively remained constant at a very lowlevel relative to benchmark.

Such fiber-optic sensors may be useful for monitoring corrosion inbridge decks and other concrete structures. Such sensors may be embeddedinto concrete decks, which may be under construction. Since the sensorsunambiguously detect the onset of corrosion, they may serve as acalibration for other corrosion detection methods.

FIG. 3 depicts a plurality of sensors 100 embedded in a concrete deck120. Placement of sensors 100 at different depths enables monitoring ofthe rate of penetration of chloride ions (shown as arrows in FIG. 3)into the deck 120. Sensors 100 may be placed at different depths tomonitor the penetration of chloride ions. The fibers may beconnectorized, and the connectors 112 may be placed in a terminal box110 at the edge of the deck, where they can be accessed for occasionalmeasurements of the reflected signal, i.e., to detect the onset of thecorrosion.

As shown in FIG. 3, any number of sensors 100 can be embedded at variouslocations in an environment of interest, such as in an environment ofrebar-enforced concrete as might be used to form a bridge deck. Theconcrete 120 may have any number of reinforcing rods (rebar) 122 runningthroughout it. The rods 122 may be made of steel. The end caps on thesensors 100 may be made of the same steel as the rods 122.

In such an environment, the onset of the corrosion of the steel rods mayindicate that sufficient chlorine has penetrated the structure to makeit unsafe. Thus, the penetration of chlorine may be detected bydetecting degradation of the end caps. End caps can be of differentthicknesses to measure the rate of corrosion. Sensors can be placed atvarious depths from the surface of the bridge deck to follow accuratelythe penetration of the chloride ions at various locations of the bridgedeck.

It is estimated that a steel cap having a thickness of about onemillimeter may resist degradation in concrete for several years. Itshould be understood however, that the sensors described herein may beused to detect degradation or wear of any material, such as glass,metal, or any other material of interest. In general, the end cap mayhave a thickness ranging from about one mil to a few millimeters, ormore, depending on the material being monitored, and the expected rateof degradation in the particular application, which may be severalhours, or several years. For example, rebar may show the onset ofcorrosion in only a few months, for example, should the surroundingchloride concentration reaches the corrosion threshold values of about0.04% of chloride by weight.

In test decks, various methods for the acceleration of the corrosion maybe applied. FIG. 4 depicts a possible placement of sensors 100 in a testconcrete deck 120 at various positions relative to a plurality of rebars122. Readings obtained from fiber-optic sensors 100 may be used tocross-calibrate other corrosion detection methods (e.g., ground radar,acoustic methods, etc). The combination of various X and Y locations(i.e., position), and of Z placements (i.e., depth), will give acomplete picture of the chloride penetration and the corrosion in thedeck instrumented in this way.

FIG. 5 depicts an example sensor system including an optical time-domainreflectometer (OTDR). An OTDR is an optoelectronic instrument used tocharacterize an optical fiber. OTDRs are well-known, and need not bedescribed herein in detail. In essence, an OTDR injects a series ofoptical pulses into the optical fiber. It also extracts, from the sameend of the fiber, light that is scattered or reflected back from pointsalong the fiber. The strength of the return pulses may be measured andintegrated as a function of time. Thus, faults in the fiber, such asbreaks, for example, may be detected and the location of such faultsdetermined.

As shown in FIG. 5, under normal operating conditions, pulses emittedfrom the OTDR will reflect back from the sensors 100A-D through thefiber network to the OTDR. The processing unit can determine from thestrength of the reflected pulses whether the material of interested hasdegraded below an acceptable level. The OTDR can determine the distancefrom which the pulses were reflected. For example, as shown, sensor 100Bmay be located a distance D from the OTDR. Under normal operatingconditions, a significant pulse will be reflected by the sensor 100B.The OTDR will determine that a significant pulse was reflected from thesensor 100B, and conclude, therefore, that the material has not yetdegraded below threshold.

If there is no significant reflection from a distance D, the OTDR candetermine that the sensor 100B has detected a material degradation. Amap as to the locations of the one or more sensors making up the networkcan be provided to the processing unit, so that the processing unitknows how far away the various sensors are placed. If an insignificantreflection, or none at all, comes from the location of any of thesensors, the OTDR can identify the locations where the material hasdegraded.

An OTDR may also be used to detect faults, such as breaks, in theoptical fibers. Suppose, for example, that a break, crack, or otheranomaly occurs in the optical fiber leading to sensor 100B, at adistance D′ from the OTDR. The anomaly will cause an unexpectedreflection. When the OTDR detects a reflection from a location in thenetwork other than where a sensor is located (e.g., the map wouldindicate no sensor at D′), the OTDR can determine that there must be afault in the optical fiber at that location. Thus, the OTDR may be usedto interrogate the optical fiber network for faults.

Sensors such as described herein may be adapted to detect the wavelengthof the reflected light. For example, signals of different wavelengthsmay be routed to different sensors. FIG. 6 depicts a sensor system basedon wavelength detection. As shown, one or more wavelength selectiveelements 130A-C may be situated at respective fiber junctions. Lightemitted from the light source may contain light having any number ofwavelengths. For example, the system may employ LEDs of variouswavelengths to provide input light.

A first element 130A may cause light having a wavelength λ1 to bedirected toward sensor 100A, while the remainder of the light passesthrough. Thus, element 130A may be transparent to the remainder of thelight. Likewise, element 130B may cause light having a wavelength λ2 tobe directed toward sensor 100B, and element 130C may cause light havinga wavelength λ3 to be directed toward sensor 100C. Elements 130A-C mayall be transparent to light having wavelength λ4, which would bedirected toward sensor 100D. The processing unit may determine from thereflectivity of light having a certain wavelength which of the sensorshas detected a material degradation.

Wavelength selective elements are well known. In particular, volumeBragg grating elements may be used as wavelength selective elements in asensor system as described herein. The use of volume Bragg gratingelements as wavelength selective elements is described in U.S. Pat. No.7,031,573, the disclosure of which is incorporated herein by reference.

It should be understood that fiber Bragg gratings 132 may be recorded inone or more of the optical fibers. In such an example, the fibers in thenetwork that have the Bragg gratings recorded therein may be used asstrain gauges. For example, it is known that, when the fiber stretches,the peak wavelength of the fiber Bragg grating shifts. Such a shift mayindicate that the fiber has been stretched, due to some condition, suchas strain on the environment in which the sensor is deployed (bridgestrain, for example). Thus, such a sensor may be used as a strain gaugeas well as a sensor for material degradation in combination.

In another example, a sensor system may be adapted to monitor for thepresence of a chemical. For example, the distal end of the optical fibermay be coated with a material that would react with the chemical ofinterest. The spectrum of the reflected light may be a function of thecolor of the material. The fiber may be placed in proximity to the areato be monitored for the chemical of interest. If the chemical ofinterest is present, then a chemical reaction with the material wouldcause the color of the material, and therefore the spectrum of thereflected light, to change. Thus, the presence of a chemical of interestmay be determined from the spectrum of the reflected light. In such asensor, the processing unit may be adapted to perform spectrum analysison the reflected light to detect changes in the spectrum thereof.

In another example, a sensor system may be adapted to detect changes inpH. For example, the distal end of the optical fiber may be coated witha pH-sensitive material. The spectrum of the reflected light may be afunction of the color of the pH-sensitive material. The fiber may beplaced in proximity to the material of interest, which may cause thepH-sensitive material to assume a color based on the pH of the materialof interest. Thus, the pH of the material of interest may be determinedfrom the spectrum of the reflected light. In such a sensor, theprocessing unit may be adapted to perform spectrum analysis on thereflected light to detect changes in the spectrum thereof.

In another example, a sensor system may be adapted to determine thedistance between the distal tip of the fiber and the material ofinterest. Such a system may be adapted to detect changes in the distancebetween the distal tip of the fiber and the material of interest. Thespectrum of the reflected light may be a function of the distancebetween the distal tip of optical fiber and the material of interest. Asthe distance between the distal tip of the optical fiber and thematerial of interest changes, so does the spectrum of the reflectedlight. Thus, such a sensor may be used to detect static changes in thedistance between the fiber and the material of interest, as well asdynamic changes, such as caused by acoustic vibrations, for example. Insuch a sensor, the processing unit may be adapted to perform spectrumanalysis on the reflected light to detect changes in the spectrumthereof.

FIG. 7 depicts an example sensor capsule 700 with wirelesscommunication. The capsule 700 is a self-contained unit that includes alight source S, a light receptor R, a processing unit P, a power source,B, which may be a battery, and an antenna A, all of which may becontained in a housing 710. As described herein, light emitted from thelight source S through an optical fiber 702 may be reflected off ofmaterial M situated at an end of the capsule 700 back through theoptical fiber 702 to the receptor R. The material may be in the form ofan end cap, or it may be coated onto the end of the capsule, or thecapsule may merely be situated in proximity to the material. The antennamay be used to communicate signals wirelessly from the capsule 700 to amonitoring station (not shown). For example, the sensor may be adaptedto text the integrity of the material periodically (e.g., once everycouple of months or so) and transmit a signal indicating whether thematerial has degraded or not. Such signals may include an identity ofthe sensor capsule. Thus, the monitoring station may be informed of thelocation of the material degradation. Such an embodiment may eliminatethe need for the sensors to be physically connected to a monitoringstation.

In another example, sensors as described herein may be used in medicalapplications. For example, such a sensor may be ingested or implanted insuch a way as to monitor biological systems, such as bone, for example,for deterioration or the onset of disease.

The invention claimed is:
 1. A sensor system, comprising: an opticalfiber; a housing through which the optical fiber extends; an opticaltime-domain reflectometer (OTDR) that injects a series of optical pulsesinto the optical fiber; and an end cap affixed to an end of the housingsuch that the pulses injected into the optical fiber are reflected offof the end cap back through the optical fiber, wherein the end cap ismade of a material of interest, and wherein the OTDR determines, basedon a distance from which the pulses were reflected, whether the materialof interest has degraded below a threshold.
 2. The sensor system ofclaim 1, wherein the OTDR extracts from the optical fiber the pulsesthat are reflected off of the end cap.
 3. The sensor system of claim 1,wherein the OTDR injects the optical pulses into an end of the opticalfiber, and extracts the reflected pulses from the same end of theoptical fiber.
 4. The sensor system of claim 1, wherein the OTDRdetermines the distance from which the pulses were reflected.
 5. Thesensor system of claim 1, wherein the OTDR determines, based on astrength of a reflected pulse, that the material of interest has notdegraded below the threshold.
 6. The sensor system of claim 1, whereinthe OTDR determines, based on a strength of a reflected pulse, that thematerial of interest has degraded below the threshold.
 7. The sensorsystem of claim 1, wherein a strength of the pulses that are reflectedoff of the end cap is measured and integrated as a function of time. 8.The sensor system of claim 7, wherein whether the material of interesthas degraded below an acceptable level is determined from a strength ofthe pulses that are reflected off of the end cap.
 9. A sensor network,comprising: a plurality of sensors; and an optical time-domainreflectometer (OTDR), wherein each of the sensors comprises a respectiveoptical fiber, a respective housing through which the optical fiberextends, and a respective end cap affixed to an end of the respectivehousing, wherein each of the end caps is made of a material of interest,and wherein the OTDR injects a respective series of optical pulses intoeach of the optical fibers, receives respective reflected pulsesreflected off of the end caps, and determines, based on respectivedistances from which the reflected pulses were reflected, a locationwithin the network at which the material of interest has degraded belowa threshold.
 10. The sensor network of claim 9, wherein the OTDRextracts from the optical fibers the pulses that are reflected off ofthe end caps.
 11. The sensor network of claim 9, wherein the OTDRinjects the optical pulses into respective ends of the optical fibers,and extracts the respective reflected pulses from the same ends of theoptical fibers.
 12. The sensor network of claim 9, further comprising amap of the respective locations of the plurality of sensors in thenetwork.
 13. The sensor network of claim 9, wherein strengths of thepulses that are reflected off of the end caps are measured andintegrated as a function of time.
 14. The sensor network of claim 9,wherein the OTDR determines the respective distances from which thepulses were reflected.
 15. The sensor network of claim 13, whereinwhether the material of interest has degraded below an acceptable levelat the location in the network is determined from the strength of thepulses that are reflected off of the end cap at that location.
 16. Thesensor network of claim 14, wherein the OTDR determines, based on thestrength of a reflected pulse received from a determined distanceassociated with a location in the network, that the material of interesthas not degraded below the threshold at that location.
 17. The sensornetwork of claim 14, wherein the OTDR determines, based on the strengthof a reflected pulse received from a determined distance associated witha location in the network, that the material of interest has degradedbelow the threshold at that location.